Zhang, Bin; (2021) Direct Ink Writing of Polycaprolactone-Based Scaffolds for Interfacial Tissue Engineering. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Engineering interface tissues (e.g. cartilage-to-bone) is a complex process requiring specific designs and organisation of materials, cells and biomolecules. Polycaprolactone (PCL) is a widely applied biomaterial in tissue engineering for its mechanical properties and biocompatibility. Direct ink writing (DIW) is a promising 3D technique to fabricate personalised scaffolds in a heat-free environment by ink deposition. However, it is challenging to develop PCL-based scaffolds using DIW as PCL is water-insoluble. This study aims to develop PCL-based inks to fabricate customised scaffolds with improved properties and functionality for DIW printing. Two types of filament-based scaffolds were designed: single lay-down angle scaffolds and a complex scaffold with layers of different lay-down angles. Finite element simulation results indicate the orthotropic effect increased when the angle decreased from 90° to 15°. Gradient strain magnitudes were achieved in a complex structure with various lay-down angles, mimicking the gradient mechanical characteristics of natural tissue. PCL-based inks were formulated by blending hydrophilic polyethylene oxide (PEO), nano-hydroxyapatite (HAp) comprising acetone and dichloromethane. Ink rheology results indicate that acetone inks had more pronounced shear-thinning behaviour than the dichloromethane inks but had inferior viscosity recovery performance. The addition of PEO and HAp had improved the ink shear-thinning behaviour, viscosity recovery performance, and the scaffold surface wettability. With the varying HAp concentration (55–85%w/w) and lay-down angle, the scaffold with 65%w/w HAp and 90° lay-down angle exhibited the highest elastic modulus and yield strength. 65% w/w HAp concentration is close to the inorganic composition of natural bone tissue. Vancomycin as a model drug was embedded in the PCL/PEO/HAp scaffold. The release behaviour of vancomycin was assessed with the in vitro dissolution test and the antibacterial activity of the printed scaffold was effectively inhibited Staphylococcus aureus in the agar diffusion test. In conclusion, the combined numerical and experimental studies reported in this thesis mainly contains: (1) the use of FEM in design structures with gradient mechanical property, (2) the development of printable PCL/PEO and PCL/PEO/HAp ink formulations, and (3) the use of DIW in fabricating the predesigned scaffold. This study demonstrates that DIW can be used to fabricate scaffold with desired properties with the proper design of ink formulation and scaffold structures. The ink formulation and methodology developed in this study can be transferred to other hydrophobic biomaterials for DIW scaffold fabrication. The gradient property of the DIW tissue scaffold could be achieved by the optimal combination of ink formulations and inner structures to match with natural interfacial tissue.

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